Ch 3 · Active Transport · O Level Biology 5090
Topic 3 of 3 in Movement into and out of Cells — O Level Biology 5090 — 53 lessons.
Introduction
Diffusion and osmosis can only move substances down a concentration gradient. Yet a root hair cell absorbs nitrate ions from soil water that contains far fewer ions than the cell already holds. This needs active transport, which uses energy from respiration to pump particles the "wrong" way. This lesson defines it, explains where it matters and ends with the comparison table examiners like to set.
Key ideas — What active transport is
Active transport is the movement of particles through a cell membrane from a region of their lower concentration to a region of their higher concentration, against a concentration gradient, using energy from respiration.
🧑🏫 Teacher's explanation
Imagine you are trying to push a heavy shopping trolley up a steep hill. It is much harder than rolling it downhill, and you have to use your own muscle power to make it happen. In a cell, moving particles from a low concentration to a high concentration is like pushing that trolley uphill; it doesn't happen naturally and requires 'muscle power' in the form of chemical energy.
📖 Definitions
🔍 Worked example — Easy
A plant root hair cell uses active transport to absorb mineral ions from the soil. The concentration of magnesium ions in the soil is 0.01 mol/dm^3, while the concentration inside the cell cytoplasm is 0.05 mol/dm^3. Describe the direction of movement and the requirement for this process.
🔍 Worked example — Standard
Compare diffusion and active transport in terms of concentration gradient and energy requirement.
🔍 Worked example — Exam-style
A researcher applies a respiratory inhibitor (a poison that stops aerobic respiration) to a layer of epithelial cells in a petri dish. Predict the effect on the uptake of glucose into the cells and explain your reasoning.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the process used by the cell to absorb these ions. [1] (b) Explain why this process requires energy. [2] [3 marks]
Key ideas — Why active transport matters
🧑🏫 Teacher's explanation
Imagine you are trying to push a heavy shopping trolley up a steep hill. It doesn't happen by itself; you have to exert physical effort to move it from a low point to a high point. In a cell, molecules like glucose or mineral ions often need to move from where there is 'not much' to where there is 'a lot'. This is moving 'against the concentration gradient'.
📖 Definitions
🔍 Worked example — Easy
A root hair cell has a mineral ion concentration of 0.02 mol/dm^3, while the surrounding soil water has a concentration of 0.005 mol/dm^3. State whether the movement of ions into the cell occurs via diffusion or active transport and explain why.
🔍 Worked example — Standard
Compare the movement of glucose in the small intestine during the early stages of digestion versus the late stages of absorption.
🔍 Worked example — Exam-style
Calculate the percentage increase in mineral ion concentration in a root hair cell if the concentration rises from 0.01 mol/dm^3 to 0.05 mol/dm^3 via active transport.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — How active transport works
Protein carriers. During active transport, protein carriers in the cell membrane move molecules or ions across the membrane. The particle binds to the carrier protein on the side where its concentration is lower; energy from respiration causes the carrier to change shape and release the particle on the side where its concentration is higher. Each carrier is specific to one type of molecule or ion, which is why cells can select what they absorb.
🧑🏫 Teacher's explanation
Imagine you are trying to push a heavy shopping trolley up a steep hill. It is much harder than pushing it downhill, and it requires a lot of physical effort from you. In a cell, moving molecules from an area where there are very few to an area where there are already many is like pushing that trolley uphill. This 'uphill' movement is called moving 'against a concentration gradient'.
📖 Definitions
🔍 Worked example — Easy
A plant root hair cell uses active transport to move nitrates from the soil into the cell. If the concentration of nitrates in the soil is 0.01 mol/dm^3 and the concentration inside the root hair cell is 0.05 mol/dm^3, describe the direction of movement and the role of the membrane.
🔍 Worked example — Standard
Calculate the percentage increase in glucose concentration in an epithelial cell if the concentration rises from 2 mmol/L to 10 mmol/L due to active transport.
🔍 Worked example — Exam-style
Compare diffusion and active transport in terms of concentration gradient and energy requirement.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe how the concentration of mineral ions inside the root hair cell can be higher than the concentration in the surrounding soil. [3] (b) State the type of movement occurring in this process. [1] [4 marks]
Key ideas — Comparing the three processes
Diffusion and osmosis are passive and continue in dead tissue; active transport stops when the cell stops respiring. Raising the temperature speeds up all three, but only active transport is affected by oxygen concentration or respiratory poisons.
🧑🏫 Teacher's explanation
Imagine you are in a crowded shopping mall. Diffusion is like people naturally spreading out from a crowded entrance into empty corridors because they have plenty of space. Osmosis is like water flowing into a sponge to balance out the dryness. Active transport, however, is like a security guard pushing people through a narrow door into an already packed room; it requires effort and energy to force things where they don't naturally want to go.
📖 Definitions
🔍 Worked example — Easy
A root hair cell uses active transport to absorb nitrate ions from the soil. The concentration of nitrate ions in the soil is 0.01 mol/dm^3, while the concentration inside the root hair cell is 0.05 mol/dm^3. Describe the direction of movement and the requirement for this process.
🔍 Worked example — Standard
Compare diffusion and active transport in terms of membrane requirement and energy requirement.
🔍 Worked example — Exam-style
A student observes that a plant's ability to absorb minerals decreases when the plant is placed in an environment with no oxygen. Explain why this occurs using the concept of active transport.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the process used by the cell to absorb these ions. [1] (b) Explain why this process is necessary for the plant and describe how it occurs. [3] [4 marks]
Worked example — Question
A student measured the uptake of nitrate ions by barley roots kept in aerated and in non-aerated nutrient solutions. After 3 hours the roots in aerated solution had absorbed 4.2 units of nitrate; the roots in non-aerated solution had absorbed 0.9 units.
🔍 Worked example — Easy
Calculate the percentage increase in nitrate uptake when moving from non-aerated to aerated solutions based on the data: 0.9 units vs 4.2 units.
🔍 Worked example — Standard
A cell membrane is partially permeable. If the concentration of glucose inside the cell is 0.8 mol/dm^3 and the concentration outside is 0.2 mol/dm^3, describe the direction of net movement of glucose via active transport.
🔍 Worked example — Exam-style
Compare the role of energy in diffusion versus active transport in a plant root cell.
⚠️ Common errors
Worked example — Model answer
(a) Movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration [B1], against a concentration gradient [B1], using energy from respiration [B1].
🔍 Worked example — Easy
Identify the three essential components required for active transport to occur in a plant root hair cell.
🔍 Worked example — Standard
A root hair cell is placed in a solution where the concentration of nitrate ions is 0.01 mol/dm^3. The concentration inside the cell is 0.05 mol/dm^3. Explain why the cell must use active transport to absorb more nitrate.
🔍 Worked example — Exam-style
An experiment is conducted where root hair cells are placed in soil water. Group A is supplied with oxygen, while Group B is placed in an anaerobic environment. Predict and explain the difference in nitrate uptake between the two groups.
⚠️ Common errors
Common mistakes
Practice
Answers